The preferred embodiments relate to an electro-pneumatic system for controlling a double-acting pneumatic actuator having a first pneumatic working chamber and a second pneumatic working chamber controllable independently from the first working chamber.
Commonly an electro-pneumatic system comprises a preliminary pneumatic control component, especially an I/P-converter, for generating a preliminary pneumatic control signal, whereby the preliminary control signal is fed to a main pneumatic control component such as an air pressure amplifier connected to an air pressure supply of, for example, 6 bar. In this exemplary case the air pressure amplifier can generate a maximum pressure of 6 bar.
Double-acting pneumatic actuators have multiple applications in the technical processing industry, especially in power generation technology. For example, typical applications of double-acting pneumatic actuators are directed towards demanding control tasks in which, for example, a valve cap in a pipeline filled with fluid must be positioned rapidly and precisely. A double-acting pneumatic actuator does not require internal springs to drive a positioning member of the actuator into a certain emergency position when the pneumatic actuator is vented. The double-acting pneumatic drive can have a positioning piston separating two pneumatic working chambers, being coupled to the valve flap and being displaced when the pressure difference between the two working chambers reaches a predefined value. Double-acting actuators have the general advantage to be particularly robust and durable while having a structure that is of simple design and cost effective.
Commonly, the double-acting pneumatic actuator is controlled by an electro-pneumatic positioner generating an electrical control signal based on an electrical actual position value, such as the position of the control valve in the technical processing plant and an electrical set point signal from a superordinate control unit, and transforms the control signal by means of an I/P-converter into a preliminary pneumatic control signal. The preliminary pneumatic control signal is then fed to an inverting amplifier which transmits opposing main pneumatic signals to the working chambers of the double-acting actuator. The pneumatic working chambers of the double-acting pneumatic actuator are pressurized against each other by a constant mean pressure value.
DE 10021 744 A1 gives an example of a double-acting pneumatic actuator in which a pressure difference between the two working chambers of the actuator is defined as the control variable. For adjusting the pressure difference the pneumatic inverting amplifier receives the control pressure generated by the I/P-converter on the basis of which the desired pressure difference is generated in the working chambers of the double-acting pneumatic actuator. The first output of the inverting amplifier receives the control pressure of the I/P-converter, wherein at the second output a respective opposite pressure is built up, complementing the control pressure at the first output to the constant supply pressure of, for example, 6 bar.
The functional necessity for the inverting amplifier results from the fact that a decrease of the first control pressure in the first working chamber requires an increase of the second control pressure in the second working chamber, wherein the main pressure value remains unchanged.
Commonly, a double-acting pneumatic actuator works by means of a supply pressure of about 6 bar, wherein a constant mean pressure value of 3.5 bar is defined between the first and the second working chamber. The double-acting pneumatic actuator can realize fast control cycles but has the disadvantage not to be sufficiently rigid because of the compressibility of the working medium air at 3.5 bar. If the application of the actuator requires a higher rigidity, hydraulic actuators are commonly used which are expensive and not suitable for all applications, especially due to the threat they pose to the environment by the hydraulic oil.